1. Field of the Invention
The invention relates to a telecommunications network comprising ground installations such as earth-based switching centers and satellites used for transmitting certain communications.
Telephone systems have made immense progress in the past quarter of a century. Today, they make use of geostationary satellites rotating at 36,000 km above transmitter points, these satellites being truly space telephone centers.
However, the advances being made are not limited to these features. New projects are beginning to emerge wherein every single individual, wherever he or she may be located, will be able to communicate with any point throughout the planet by means of small, portable transceiver sets.
Indeed, telecommunications systems that use moving constellations of orbiting satellites are now being seriously considered for telephony and message systems.
Telecommunications systems such as these are based on the use of small-sized satellites placed in low orbits.
All the satellites of a constellation can be used to set up a telecommunications network providing for an exchange of information between fixed or mobile users (ships, aircraft and cars), distributed over a defined services zone (for example USA or Europe) or throughout the planet.
This type of communications system should make it possible to cope with the growing need for new services such as links among portable terminals, car phones and services related to message systems.
2. Description of the Prior Art
Two major approaches are currently envisaged for the implementation and distribution of services such as these:
local transmission limited to a country or a group of countries, for example USA, Europe etc.; PA1 worldwide transmission, every point of the planet being covered by the system. PA1 enabling the conveying of all the traffic; PA1 minimizing the additional complexity entailed in
Throughout the following description, the expression "links between satellites" is understood to mean links that are set up by going through a station on the ground. The expression "inter-satellite" links is understood to mean direct links between satellites.
FIG. 1 shows the structure of a constellation telephone network, enabling the local transmission of calls sent by telecommunication terminals 3, whether portable or non-portable.
The present structure used to obtain a local telephone service is based on a constellation of satellites 4 ensuring the visibility of the service zone 1 associated with a network of earth stations 2.
The earth stations 2 enable the management or control of communications, the management or control of the hand-over process (i.e. the continuity of the links) and the interconnection with existing earth-based telecommunications networks.
However, the size of such a network is dependent on the configuration of the network of stations on the ground. The service is related to the layout of a station on the ground and gives this network a local character.
FIG. 2 shows a structure of a constellation telephone network enabling worldwide transmission. In a network such as this, inter-satellite links are implemented to convey communications via the space network, in order to limit the network of earth stations. With the possibility of conveying communications via the space network, the service zone of the telecommunications system no longer depends on the layout of the network on the ground and depends solely on the characteristics of visibility of the constellation.
The following publications provide a description of current knowledge on the subject:
[BES 78] D. C. BESTE, "Design of Satellite Constellations for Optimal Continuous Coverage", IEEE Trans. on Aerospace and Electronic Systems, Vol. AES-14, No. 3, pp. 466-473, May 1978.
[BAL 78] A. H. BALLARD, "Rosette Constellations of Earth Satellites", IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-16, No. 5, September 1980.
[BIN 87] R. BINDER, S. D. HUFFMANN, I. GURANTZ, P. A. VENA, "Crosslink Architectures for a Multiple Satellite System", Proceedings of the IEEE, Vol 75, No 1, pp. 74-82, January 87.
[IRI 90] Application of Motorola Satellite Communications, Inc. "IRIDIUM, A Low Earth Orbit Mobile Satellite System", before the Federal Communications Commission, Washington DC, December 90.
[KAT 92] J. KANIYIL, J. TAKEI, S. SHIMAMOTO, Y. ONOZATO, T. USUI, I. OKA, T. KAWABATA, "A Global Message Network Employing Low Earth Orbiting Satellites", IEEE Selected Areas in Communications, Vol. 10, No. 2, pp. 418-427, February 92.
[MAR 91] G. MARAL and J. J. DERIDDER, "Basic Concepts of Low Earth Orbit Satellite Systems for Communications", Telecom Paris-ENST, 1991.
[RID 86] L. RIDER, "Analytic Design of Satellite Constellations for Zonal Earth Coverage Using Inclined Circular Orbits", The Journal of Astronomical Sciences, Vol. 34, No. 1, January-March 1986, pp. 31-64;
[RID 87] L. RIDER and W. S. ADAMS, "Circular Polar Constellations Providing Single or Multiple Coverage Above A Specified Latitude", The Journal of Astronomical Sciences, Vol. 35, No. 2, April-June 1987, pp. 155-192.
[WAL 77] J. G. WALKER, "Continuous Whole-Earth Coverage by Circular-Orbit Patterns", Technical Report 77044, Royal Aircraft Establishment, Mar. 24, 1977.
Among all the systems described in these publications, reference may be made to the project "IRIDIUM" (registered mark) which is designed for worldwide transmission .and, in its latest development, is based on the use of 66 small satellites moving in low earth orbit, linked with earth-based switching centers distributed throughout the world.
According to this project, calls sent out by a portable terminal are picked up by a satellite and conveyed either to other satellites by inter-satellite links or, in the case of a local call, to the earth station seen by this satellite.
For this purpose, each satellite has equipment enabling the setting up of a number of inter-satellite links greater than or equal to four. This equipment is heavy and complex, for it comprises four to six antennas and mechanisms to control the orientation of these antennas.
Given that 90% of the traffic is local, to implement the 10% that constitutes international traffic, the complexity entailed in setting up inter-satellite links according to this approach would appear to be prohibitive.
The present invention makes it possible to overcome these drawbacks.
Indeed, the present invention enables the associating of the following two concepts:
the transmission of worldwide communications as compared with a local transmission system.
The geometrical structure of the network proposed according to the invention can also be used to simplify the management or control of the telecommunications and considerably minimize the problems of routing caused by a constellation structure with a large number of inter-satellite links.